The present invention relates to a relay controller for driving an excitation winding of a relay, and a relay device for switching loads.
When relays are used, high-side or low-side switches that connect an excitation winding of the relay to the operating voltage are used. In this case, the term high-side or low-side identifies the position of the switch relative to the load, which in this case is the excitation winding of the relay. A high-side switch is connected by one terminal to a battery, and a low-side switch is connected by one terminal to a reference potential, usually earth. A relay with a high-side switch is illustrated in FIG. 1. The current through the excitation winding is limited by the coil resistance of the excitation winding for example in automotive applications. The disadvantages of such arrangements are the high current consumption after switch-on, the high costs of the excitation winding and the high inductance of the excitation winding. The high inductance of the excitation winding, which arises as a result of the many windings with a thin wire having a high impedance, makes it more difficult for commutation of the relay to be effected, and a slow drop-out of the relay operating contacts of the relay is the consequence. The slow drop-out of the secondary side of the relay can enable sparking to occur at the relay operating contacts of the relay. This sparking considerably impairs the service life of the relay.
A current-saving relay driving system reduces the current after the pull-in of the relay armature, that is to say shortly after switch-on, in order thus to reduce the power consumption of the switched-on relay. Such a circuit arrangement for the operation of a relay is disclosed in DE4410819. In DE4410819, a switch T1 bridges a holding resistor R4, which sets the holding current of the excitation winding of the relay. As a result of the bridging of the resistor R4, a higher pull-in current is available at the first moment of switching on the excitation winding.
For commutation purposes, a commutation voltage has to be applied counter to the current direction via the excitation winding; the higher said commutation voltage, the more rapidly the energy of the excitation winding is reduced and the faster the commutation becomes. A diode reverse-connected across the excitation winding can be used for commutation purposes, such that the commutation current can flow through the then conducting diode, as is illustrated in FIG. 3. The diode has the disadvantage that a forward-biased diode permits only a low commutation voltage across the excitation winding, with the result that the commutation takes place slowly. As is illustrated in FIG. 3, a zener diode can also be used for commutation purposes, said zener diode being connected to the excitation winding of the relay in such a way that the commutation current can flow through the zener diode undergoing breakdown. A zener diode has the disadvantage of a very high power loss. Moreover, a high proportion of energy is drawn from the battery and converted in addition to the energy in the winding in the switch.
As illustrated in FIG. 3, a resistor can also be used for commutation purposes, such that the commutation current can flow through the resistor connected in parallel with the excitation coil. A resistor permits a high voltage on the excitation winding. The higher the voltage on the excitation winding is chosen to be, the more rapidly the excitation current decreases. The relay contacts open more rapidly in the case of a high commutation voltage at the excitation coil than in the case of a low commutation voltage. Rapid opening of the relay contacts reduces erosion of the relay contacts. A resistor has the disadvantage that a high voltage pulse arises shortly after turn-off, which pulse can only be controlled with expensive high-voltage semiconductor switches. A resistor has the further disadvantage that current flows through the resistor when a relay is switched on.
In automobiles, in particular, in which the petrol consumption is directly dependent on the current requirement of the electronics used, solutions which reduce the current consumption of the electronics and hence the CO2 emissions of the automobile, are inexpensive to manufacture and have a long service life are becoming important.